1. Field of the Invention
The present invention relates to a method and apparatus for forming a silicon nitride film on a target substrate, such as a semiconductor wafer, and particularly to a film formation technique used for a semiconductor process. The term “semiconductor process” used herein includes various kinds of processes which are performed to manufacture a semiconductor device or a structure having wiring layers, electrodes, and the like to be connected to a semiconductor device, on a target substrate, such as a semiconductor wafer or a glass substrate used for an LCD (Liquid Crystal Display) or FPD (Flat Panel Display), by forming semiconductor layers, insulating layers, and conductive layers in predetermined patterns on the target substrate.
2. Description of the Related Art
In manufacturing semiconductor devices for constituting semiconductor integrated circuits, a target substrate, such as a semiconductor wafer, is subjected to various processes, such as film formation, etching, oxidation, diffusion, reformation, annealing, and natural oxide film removal. Jpn. Pat. Appln. KOKAI Publication No. 2004-6801 discloses a semiconductor processing method of this kind performed in a vertical heat-processing apparatus (of the so-called batch type). According to this method, semiconductor wafers are first transferred from a wafer cassette onto a vertical wafer boat and supported thereon at intervals in the vertical direction. The wafer cassette can store, e.g., 25 wafers, while the wafer boat can support 30 to 150 wafers. Then, the wafer boat is loaded into a process container from below, and the process container is airtightly closed. Then, a predetermined heat-process is performed, while the process conditions, such as process gas flow rate, process pressure, and process temperature, are controlled.
In recent years, owing to the demands of increased integration and miniaturization of semiconductor integrated circuits, it is required to alleviate the thermal history of semiconductor devices in manufacturing steps (in consideration of the heat resistance property of film formation materials), thereby improving the characteristics of the devices. For vertical processing apparatuses, it is also required to improve semiconductor processing methods in accordance with the demands described above. For example, recently, there is a tendency to use film formation gases easy to thermally decompose at a relatively low temperature, which are known to include, e.g., HCD (hexachlorodisilane) and DCS (dichlorosilane).
For example, where HCD is used to form an insulating film of silicon nitride (SiN), HCD gas and ammonia gas are supplied at the same time into a process field within a reaction container being vacuum-exhausted. Further, the process field is heated at, e.g., about 500° C. to cause thermal decomposition reaction of the process gas, thereby forming a silicon nitride film on a semiconductor wafer by CVD (chemical vapor deposition). Silicon nitride films thus formed are frequently used as, e.g., a contact stopper film. Where HCD gas is used in a film formation process, the process can be performed at a relatively low temperature. In this case, there is an advantage in that film materials formed in previous steps suffer less thermal damage. However, the present inventors have found that silicon nitride films thus formed may generate particles.